The present invention relates to method for manufacturing gears in general, and particularly to a method for manufacturing net formed (straight bevel) gears having a predetermined tooth contact surface.
Straight bevel gears are used for many automotive applications, such as differential gearing. Conventionally, the straight bevel gears of the prior art have tooth surfaces designed in the form of involute or octoid. Substantially the whole tooth surfaces of the conventional straight bevel gears are adapted to engage a tooth surface of a conjugated gear. Knowledge of the exact pattern of the tooth contact area is extremely important for engineers designing various gear mechanisms, such as differentials, because the size and position of the tooth contact area determines a gear tooth load and influences the operation of the gear mechanism. However, the prior art provides no means to predetermine the gear tooth contact area of the conjugated gears.
Currently, gear manufacturers employ a number of various methods for making gears. Recently, forging technology has achieved tremendous development and become very popular. Compared to machining, forging has its advantages of improving the product quality by achieving a high stiffness in the material without cutting the material flow lines, good surface condition at the formed part, and work hardening that increases wear resistance, and quick cycle time suitable for mass production.
Forging die design is essential for the manufacturing of the gear because it determines the shape of the gear and, thus, a meshing gear contact area and performance of the gearing incorporating this gear. Currently, forging dies for straight bevel gears are generated by a gear cutter and gear generating machine. Thus, gear tooth contact surface of the straight bevel gear is determined by a profile of the cutter and gear machine setting. It is well known to those skilled in the art that the tooth contact surface of the gear made by cutting on a gear-cutting machine is very difficult to control due to rigidity of the cutting machine and cutter errors. Therefore, it is practically impossible to control the gear tooth contact surface. The resulting gear tooth contact surface may be of any shape and configuration.
For forged gears that use cut gears as an electrode for making a gear forging die, problems, similar to described above, exist.
The present invention alleviates the drawbacks of the prior art.
The present invention provides a method for designing gear tooth profiles having a predetermined gear tooth contact area (both size and location). An important element of this invention is a method for modifying a conventional gear tooth surface for determining a new modified gear tooth surface having the predetermined gear tooth contact area. This method includes the steps of: (a) defining a conventional tooth working surface, (b) defining the predetermined gear tooth contact area, and (c) modifying the conventional tooth working surface outside the predetermined gear tooth contact area to determine a modified tooth working surface having the predetermined gear mesh contact area
The conjugated gears manufactured in accordance with the present invention provide reduced stress concentration and noise level, improved durability over the current designs, and are less sensitive to misalignment. The present invention discloses mathematical equations determining the position of any point on the modified surface of the gear tooth.
In accordance with the preferred embodiment of the present invention, the bevel gear is manufactured by die forging. The method for manufacturing the gear-forging die is contemplated in the present invention. This method includes the steps of: (a) defining a conventional tooth working surface, (b) defining the predetermined gear tooth contact area, (c) modifying the conventional tooth working surface outside the predetermined gear tooth contact area to determine a modified tooth working surface having the predetermined gear mesh contact area, (d) designing a gear having the gear tooth surface as determined in the previous step; (e) designing an electrode for an electric discharge machining (EDM) on a CAD/CAM device using numerical data from the step (d), (f) manufacturing the electrode for the electric discharge machining, and (g) manufacturing the forging die employing the electric discharge machining process using the electrode. Thus, the present method eliminates the need in the gear cutter and gear cutting machines, and the forging die manufactured in accordance with the present method more closely corresponds to the profile of the originally designed gear.